Gas-powered furnace systems such as water heaters commonly are millivolt systems in which a thermo-generator or thermopile supplies low-voltage power for operating a gas valve. The thermo-generator typically has wires of dissimilar metals that produce a voltage when heated together in a furnace pilot flame. A millivolt gas valve typically has a solenoid or magnetic coil that can be actuated to open the valve and keep it open for as long as needed. When the coil is actuated, it “pulls in” a valve member from an opening in the valve so as to allow the flow of gas through the valve. When current to the coil is stopped, the valve member returns to its normal position and thus closes the valve.
A magnetic actuator for a gas valve must be strong enough to open the gas valve to a gas port and also to hold the valve open for the duration of a call for heat. A magnetic actuator typically uses about twice as much power to open a gas valve as it does to keep the valve in an open position. Thus the coil needs to be large enough to be able to utilize enough power from the thermo-generator to open the gas valve, even though only half as much power typically is needed to hold the valve open. Space requirements and costs, however, increase with coil size.